1. Field of the Invention
The present invention relates to an apparatus and methods for reducing the stray magnetic fields created by a cathode ray tube (CRT) visual display, and, in particular, to apparatus and methods for passively inducing an opposing magnetic field to reduce the stray magnetic field emitted from a CRT enclosure.
2. Art Background
CRTs are commonly used in televisions and in connection with computers as visual display devices. As is well known in the field, the CRT operates by producing a beam of electrons, which is then scanned across a fluorescent screen. The scanning of the electron beam is accomplished by a deflection circuit controlling an electro-magnet known as the yoke. The yoke surrounds the CRT just before the CRT flares out to form the enlarged portion of the CRT containing the fluorescent screen. When an electric current is passed through the conductive windings in the yoke, a magnetic field is created which will deflect the electron beam as the beam passes through the yoke region. By controlling the current in the yoke windings, the electron beam may be deflected in any desired direction, and thus scanned over the CRT screen to produce an image. However, in addition to creating the magnetic field necessary to scan the electron beam, the yoke creates a wide ranging stray magnetic field. This stray field, although not affecting the CRT whose yoke created the field, can deleteriously affect other CRTs or instruments sensitive to magnetic fields.
At present, a common method to reduce the stray magnetic fields produced by the yoke is to add bucking coils in series with the yoke. These coils, also known as compensating coils, are physically formed so as to produce a magnetic field to oppose the magnetic field produced by the yoke. Although the total magnetic field outside of the CRT enclosure in fact is diminished, several disadvantages become apparent. First, the current necessary to create a functional compensating magnetic field reduces the efficiency of the entire deflection circuit. In a typical compensation case, the bucking coil current typically is of the order of fifteen amperes, requiring a large power supply. Second, in order to supply the bucking coils with sufficient current to form a compensating field while in series configuration with the yoke, the deflection voltage on the yoke itself must be increased, which affects CRT picture quality. Third, because the bucking coils are in series with the yoke, any change in the bucking circuit can directly affect the CRT picture quality. And fourth, magnetic suppression by the bucking coils may inadequately prevent magnetic radiation emission from CRT enclosures, particularly in radiation sensitive applications.
In view of the foregoing, one objective of the present invention is provide an uncomplicated apparatus for, and method of, reducing the stray magnetic fields emitted from CRT enclosures.
Another objective of the present invention is to provide a less costly apparatus for, and method of, reducing CRT stray magnetic fields. Using the teachings of the present invention, a compensation circuit is available at significant savings compared to prior art embodiments.
Yet another objective is to disclose a more effective apparatus for, and method of, reducing CRT stray magnetic fields. As taught by the present invention, CRT stray magnetic fields are suppressed more effectively than using the teachings of the prior art. Moreover, suppression profiles can be optimized for particular environments.